Macromolecules 2009, 42, 2729-2736
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Direct Analysis of the Ion-Hopping Process Associated with the R-Relaxation in Perfluorosulfonate Ionomers Using Quasielastic Neutron Scattering† Kirt A. Page,*,‡ Jong Keun Park,§ Robert B. Moore,§ and Victoria Garcia Sakai|,⊥,# Polymers DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Department of Materials Science and Engineering, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed July 8, 2008; ReVised Manuscript ReceiVed January 22, 2009
ABSTRACT: This work demonstrates the ability of quasielastic neutron scattering (QENS) to measure the dynamics associated with counterions in perfluorosulfonate ionomers (PFSIs). PFSI membranes were prepared by neutralizing with hydrogenated alkyl ammonium counterions. Counterion dynamics were measured using the High-Flux Backscattering Spectrometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). Long-range mobility of the counterions was closely linked with the R-relaxation in these materials measured by dynamic mechanical analysis (DMA). The counterion motions in the membrane were found to follow a mechanism of random jump-diffusion within a confined spatial region with diffusion coefficients on the order of 10-7 cm2 s-1. These data are presented along with variable temperature X-ray scattering investigations of the melting behavior of these materials. Altogether, the data presented here show the link between the onset of long-range counterion mobility and the mechanical properties of these materials. These data provide further fundamental understanding of the link between electrostatic interactions and dynamics in PFSI materials.
Introduction Ionomers are an interesting class of polymeric materials that contain ionic comonomer units distributed along the polymer backbone and have been the focus of many studies over the last several decades.1-17 The physical properties of these materials are dictated by the Coulombic interactions between the ion-pairs along the backbone. These electrostatic interactions lead to the formation of stable ionic associations that behave in many ways as cross-links. In particular, the relaxation behavior of these materials is greatly affected by the associations, and the resulting aggregates (multiplets), and has been the focus of several rheological studies.11-14,16,17 Several studies have shown that the rheological behavior of these materials can be altered simply by changing the degree of neutralization or through the choice of counterion, factors that can be expected to influence the strength of the associations.7 In particular, and pertinent to later discussion, early patent literature describes the neutralization of ionomers with alkyl ammonium ions and subsequent changes in the rheological properties.18 In addition, Weiss et al.15 observed a decrease in the viscosity of alkylamine neutralized polystyrene ionomers with an increase in the bulkiness of the counterion and found that electrostatic interactions predominate for smaller counterions, while plasticization (increase in the segmental dynamics) is more important as the counterion becomes sufficiently bulky. These, and other studies, † Reported data error bars and value uncertainties represent one standard deviation as the estimated standard uncertainty of the measurements and the fits, respectively. * To whom all correspondence should be addressed. E-mail:
[email protected]. ‡ Polymers Division, National Institute of Standards and Technology. § Department of Chemistry, Virginia Polytechnic Institute and State University. | Current address: ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. ⊥ NIST Center for Neutron Research, National Institute of Standards and Technology. # Department of Materials Science and Engineering, University of Maryland.
show that by changing the overall number and strength of the electrostatic interactions, a large degree of control can be exerted over the relaxation processes in these materials. Although it is beyond the scope of this discussion, there have been several investigations of the relaxation behavior of ion containing polymers.1,10 In short, two “glass transition” temperatures (Tg) have been reported for ion containing polymers: one attributed to the Tg of the “matrix” chains removed from the ionic aggregates and the other attributed to the Tg of the “cluster” domains, or chains in the vicinity of an ionic aggregate that have a restricted mobility due to decreased conformational entropy. Our work has shown that for perfluorosulfonate ionomers (PFSIs) this description of the relaxation processes is not entirely accurate.19,20 However, there are similarities in our description of the relaxations in PFSIs and the classical ionomer description. The two predominant relaxation processes governing the melt rheological behavior of ionomers, in general, are (1) the terminal relaxation time of the polymer chains and (2) the average lifetime of the ionic associations.13 Ultimately, the terminal relaxation time is dictated by the time that an ion pair resides in an aggregate before “hopping” to another aggregation site. This process has been termed “ion-hopping”, and τ is the ionhopping time.2,4,9,11-14,21 The relative time scale of these two processes has been studied using rheology and cation diffusion measurements. The onset of this ion hopping process has been experimentally observed for both styrenecarboxylates and styrenesulfonates.9,21,22 Ion-hopping has been identified as a principle phenomenon in the mechanism for the cluster “glass transition”, or R-relaxation process, for these materials. Essentially, the presence of appreciable ion-hopping means the ionic aggregates are dynamic and the rigidity of the system is reduced because of the labile nature of these associations.1 The concept of a temperature at which the electrostatic cross-links (i.e., multiplets) become thermodynamically unstable was proposed by Eisenberg in a theoretical description of clustering in ionomers in 1970.3 The “ion-hopping temperature”, of
10.1021/ma801533h CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
2730 Page et al.
Eisenberg’s, is defined as the temperature at which the multiplet becomes thermodynamically unstable and the elastic forces of the chain are balanced with the electrostatic forces of the aggregates.3 Our previous work has drawn upon the concept of ion-hopping to explain the molecular origins of the relaxation processes in PFSIs.19,20 It should be clarified that we are careful not to definitively assign the R-relaxation to a pure glass transition temperature, but rather to a transition from a state exhibiting static electrostatic cross-links to a state exhibiting dynamic electrostatic cross-links brought about by the onset of ion-hopping. Furthermore, both the R-relaxation and the ionhopping phenomenon are recognized to be frequency dependent, dynamic processes. Recent simulations of ionomer self-assembly by Kumar and co-workers23 provide some basic insights into the nature of these fluids. Upon cooling, there is first a transition where the counterions pair with the ions on the chain backbone to form small ion-multiplet structures that remain quite mobile. The character of these ion pairs in “primitive model” ionic fluids has been shown to depend on ion valence and ion size asymmetry,24,25 forming dipolar, or multiple, low energy structures depending on these asymmetry factors. The longrange interactions of the small ion-multiplet structures lead to the supermolecular assembly of the small ion-multiplets (ribbonlike structures in the simulations of Kumar and co-workers) in a well-defined order-disorder transition upon further cooling characterized by a maximum in the specific heat and a sharp drop in the mobility of counterions that have been incorporated into the self-assembled structures. Kumar and co-workers suggest that this self-assembly transition can be identified with the “cluster Tg” of Eisenberg.3,4,23 The second, lower Tg, is naturally attributed to the conventional Tg of the polymeric structures with the large-scale supermolecularly assembled ionomer multiplet structures. The description put forth by the simulations of Kumar et al.23 is congruent with earlier descriptions of the relaxation processes for PFSIs. It is apparent that the “self-assembly” of the ion pairs into supermolecular structures that they observe upon cooling corresponds to the same process involved in destabilization of the electrostatic network upon heating described in earlier work. Nafion is a widely examined PFSI and the focus of our work.26
The perfluoroether side-chains containing the ionic, sulfonate groups have been shown to organize into aggregates, thus leading to a nanophase-separated morphology where the ionic domains, termed clusters, are distributed throughout the nonpolar poly(tetrafluoroethylene) (PTFE) matrix. This morphology gives a characteristic scattering peak in small-angle X-ray and neutron scattering centered at Q ) 0.20 Å-1 (Q ) 2π/d, where d is the length scale of motion).26 The membranes used in this study have been neutralized with tetramethylammonium (TMA+) and tetrabutyl ammonium (TBA+) counterions according to previously published methods.19,20 The size, or polarizability, of the counterion influences the strength of the electrostatic network, thus changing the chain dynamics and, in turn, the mechanical properties.19,20 Our earlier studies of these materials correlating dynamic mechanical analysis (DMA) with a variety of other techniques show that the R-relaxation of these materials is due to the onset of long-range mobility of both the main- and side-chains, which
Macromolecules, Vol. 42, No. 7, 2009
is facilitated by a profound weakening of the electrostatic interactions within the ionic aggregates. At temperatures in the vicinity of the R-relaxation (TR), a significant destabilization of the electrostatic network may be observed, which results in the activation of a dynamic network facilitated through the ionhopping process described earlier. In contrast, the β-relaxation (Tβ) was associated with the onset of segmental motions (principally backbone motions) within the framework of a static physically cross-linked network of chains.19,20 The present work uses quasielastic neutron scattering (QENS) to investigate the correlations between counterion dynamics and the bulk mechanical relaxations in alkyl ammonium neutralized PFSIs in order to obtain a better understanding of the physical basis of the relaxation processes in these materials. QENS provides a direct measure of counterion dynamics, making it possible to calculate time-scales associated with the motions of the ions and ion diffusion coefficients for temperatures above the R-relaxation. QENS measures the dynamic structure factor in the frequency domain, thus providing information about the motions of atoms, or molecules, on a time scale of 10-8 - 10-13 s. QENS is sensitive to the motion of hydrogen atoms, which have a significantly larger incoherent scattering length than other atoms. In the systems presented here, only the counterions contain hydrogen, thus allowing us to isolate the dynamics of the counterions. We use QENS to study the counterion dynamics, and thus directly confirm and characterize the ion-hopping process. Experimental Section Materials. Nafion 117 (1100 EW, sulfonic acid form) films were obtained from E. I. du Pont de Nemours & Co. The tetramethyl(TMA+), tetraethyl- (TEA+), and tetrabutylammonium (TBA+) counterions were obtained from Aldrich in the form of hydroxides dissolved in either water or methanol. All other reagents were obtained from Aldrich and used without further purification. PFSI Sample Preparation. The PFSI membranes were cleaned by refluxing in 4 mol/L methanolic H2SO4 for ca. 12 h. These H+form membranes were then washed with deionized (DI) water to remove excess acid. The TMA+, TEA+, and TBA+ form samples were prepared by soaking the H+-form membranes in a 5 M excess of solutions of the appropriate alkylammonium hydroxide. The neutralized membranes were then thoroughly rinsed of excess alkylammonium hydroxide and dried in a vacuum oven at 70 °C, overnight. Quasielastic Neutron Scattering. The QENS experiments were performed on the High-Flux Backscattering Spectrometer (HFBS)27 at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). Films of 145 µm thickness (5.5 cm × 8 cm), used to achieve ∼90% neutron transmission through the sample and avoid multiple scattering, were loaded into annular, thin-walled (500 µm) aluminum cells and mounted to a closedcycle refrigerator unit. Initial temperature scans were performed with the instrument in the so-called fixed window mode, where only the elastic scattering is measured, over a temperature range from -223 to +277 °C, at a heating rate of 1 °C/min and over a momentum transfer (or scattering vector, Q) range from 0.25 to 1.75 Å-1. The raw data were normalized to the beam monitor. In general, such scans are comparable to thermal techniques such as differential scanning calorimetry (DSC) and clearly show relaxations in the system. Further information regarding the dynamics of the counterions was obtained from measuring the dynamic structure factor, S(Q,ω), at different temperatures using an energy range (17 µeV (resolution of 0.85 µeV) and over a Q range 0.25-1.75 Å-1. The data were reduced by normalizing intensity of beam monitor and corrected for detector efficiency. The instrument resolution function was measured by performing a dynamic scan on the sample at -233 °C. At this temperature, we assume that all motions are “frozen” and that the scattering is purely elastic. The data reduction and the analysis, including peak deconvolution were carried out
Macromolecules, Vol. 42, No. 7, 2009 using DAVE [Data Analysis and Visualization Environment], a software package developed at the NCNR.28 Small-Angle X-ray and Neutron Scattering (SAXS/SANS). Variable temperature (VT), time-resolved small-angle X-ray scattering was performed at the Brookhaven National Laboratory on the Advanced Polymer Beamline (X27C) at the National Synchrotron Light Source. The incident X-ray beam was tuned to a wavelength of 0.1366 nm and the sample to detector distance was 85 cm. The two-dimensional scattering images were recorded using a Mar CCD camera with an intensity uncertainty on the order of 2%. The Nafion samples were heated in a sample chamber in the X-ray beam at a heating rate of 5 °C/min with an uncertainty of ( 1 °C while acquisition times were kept to 1 min and data were recorded from 50 to 300 °C. Intensity versus pixel data were obtained from the integration of the 2-D images using the POLAR software developed by Stonybrook Technology and Applied Research, Inc. The relationship between pixel and the momentum transfer vector Q was determined by calibrating the scattering data with a silver behenate standard. All scattering intensities were corrected for transmission, incident beam flux, and background scatter due to air and Kapton windows. The SANS measurements were carried out on the NG7 beamline at the NCNR. The NG7 beamline includes a high-resolution 2D neutron detector (65 × 65) cm2 and focusing refractive lenses. The instrument employs a mechanical velocity selector as a monochromator and a circular pinhole collimator. The SANS intensity, I, was recorded as a function of the magnitude of the scattering vector Q (Q ) 4π sin(θ/2)/λ, where θ is the scattering angle and λ is the neutron wavelength, equal to 6 Å) and corrected for film thickness and background. The detector angle was set at 0°, and the sampleto-detector distance was set to 1 m, 4.5 m, and 13 m to cover the Q range 0.003-0.6 Å-1. These settings enable one to probe structural features in materials ranging from approximately 10 to 2000 Å. The intensity was corrected for background scatter, sample thickness, and detector dark current using software developed at the NCNR.29 Wide-Angle X-ray Diffraction (WAXD). Synchrotron wideangle X-ray diffraction (WAXD) was performed at the Argonne National Laboratory on the DND-CAT (5-ID) beamline at the Advanced Photon Source. The wavelength of the incident X-ray was 0.82656 Å and the sample-to-detector distance was 236 mm. The samples were heated from 50 to 300 °C using a Linkam heating stage (THMS600) at a heating rate of 5 °C/min (temperature stability
